郯庐断裂带张八岭隆起北段的左旋走滑挤压变形及其40Ar/39Ar定年

郯庐断裂带张八岭隆起北段,自西向东分别出露北北东向的韧性滑脱变形带、脆-韧性过渡带和脆性的前陆褶断带。韧性滑脱变形带内的张八岭群片岩,广泛发生了低绿片相背景下的糜棱岩化。其中呈现为平缓的糜棱面理和近南北向的矿物拉伸线理。显微构造及石英C轴组构分析显示,该韧性滑脱带一致为上盘向南的运动。该带以东依次变为上盘向南南东→南东的逆冲活动,总体上为左旋走滑挤压变形带。张八岭群所在的韧性变形带为深部陡立走滑构造与浅部脆性构造之间的滑脱变形带,其间的差异走滑变形,造成了该滑脱层在总体向北运动中出现上盘向南的剪切变形。对6处张八岭群片岩中15个不同粒级白云母的40Ar/39Ar定年指示,变形发生在(236.2±0.5)~(238.0±0.4)Ma的中三叠世晚期。这表明郯庐断裂带的左行平移发生在华北与华南板块碰撞的深俯冲阶段,起源于陆内转换断层。

Sinistral transpressive deformation in the northern part of Zhangbaling uplift in the Tan-Lu fault zone and its 40Ar/39Ar dating.

Ductile detachment deformation belt, brittle-ductile transition belt and brittle foreland fold-fault belt are exposed on the northern part of Zhangbaling uplift in the Tan-Lu fault zone from west to east. The Zhangbaling schists in the ductile detachment deformation belt are widely mylonitized under low greenschist facies. The belt shows gentle mylonitic foliation and nearly N-S trending mineral elongation lineation. Analyses of microstructures and quartz C-axis fabrics demonstrate that the ductile detachment belt has experienced top-to-south motion. The deformation motion changes into top-to-SSE then top-to-SE thrusting, showing a sinistral transpressive deformation zone. The ductile deformation belt exposed in the Zhangbaling group is a detachment deformation belt between deeper, steep strike-slip structures and shallower, brittle structures. The differential deformation between the different levels caused top-to-south shear motion under a general northward movement of the fault block. 40Ar/39Ar dating of muscovite separated from the Zhangbaling schists with different grain sizes indicates that the deformation took place in a range from (236.2±0.5) Ma to (238.0±0.4) Ma, i.e. the latest Middle Triassic. It is suggested that the sinistral displacement of the Tan-Lu fault zone happened in a continental, deep subduction stage of the North and South China plate collision, and was derived from intracontinental transform faulting.